1. Field of the Invention
This invention relates to conjugate polycarbonate fibers and fibrous sheets composed of such polycarbonate fibers. Further, the invention relates to non-woven fabrics made of conjugate polycarbonate fibers which fabrics can be very effectively used as electric insulating materials for various OF cables for ultra high voltages, or capacitors, because they do not undergo any change of form, such as swelling, dissolution and crazing when in contact with insulating oils such as mineral oils and synthetic oils and they have a compact structure characterized by a Gurley porosity of at least 500 seconds per 100 cc.
The term polycarbonate used in this invention means a linear polymer containing carbonic acid ester linkages ##STR1## recurring periodically in the molecule. These polycarbonates are known substances, per se. They can be synthesized by, for example, the phosgene method in which phosgene is introduced into a 4,4'-dihydroxydiaryl alkane, such as bisphenol A, in the presence of an aqueous solution of an alkali and a solvent, or the ester exchange method in which the 4,4'-dihydroxydiaryl alkane is reacted with a diester of carbonic acid at a high temperature under a reduced pressure.
In the instant specification, the expression "a Gurley porosity of at least 500 seconds per 100 cc" is used for indicating the compactness of the structure. This expression means that the sample has an air permeability of at least 500 seconds per 100 cc as measured according to the method of JIS P-8117 for testing the air permeability of papers and cards using a tester of the B type (Gurley's densometer).
2. Description of the Prior Art
Polycarbonate resins have heretofore been used in the form of films as insulating materials for electric equipment such as condensers and transformers because they possess excellent heat resistance and dielectric characteristics, and they also are suitable as electric insulating materials. Polycarbonate resin films are advantageous in that the dielectric loss is small, but they are defective in that their oil immersion is very poor because of their smooth surface, and that their dielectric strength abruptly decreases with an increase of the thickness.
According to this invention, these defects of polycarbonate films are overcome by preparing polycarbonate fibers from polycarbonate resins and forming the fibers into fibrous sheets such as non-woven fabrics, papers and woven fabrics. When such fibrous sheet is used as an insulating material of an OF cable for 500 KV or 750 KV, the sheet should have a dielectric strength (impulse breakdown strength) of at least 120 KV/mm. For this purpose, the fibrous sheet should have a compact structure and in view of the oil immersion the fibrous sheet sould have a porous structure preferably a non-woven fibrous sheet structure. In short, the fibrous sheet should be a compact non-woven fabric having a uniform fine porous structure characterized by a Gurley porosity of at least 500 seconds per 100 cc.
More specifically, if the Gurley porosity is lower than 500 seconds per 100 cc, the fibrous sheet cannot be used because of its poor dielectric strength, and if the fibrous sheet does not have a fine porous structure like film it cannot be used because of a poor oil immersion. Therefore it was found that a non-woven fabric having a Gurley porosity of 500-50,000 seconds per 100 cc is required for this purpose.